How to use Gnupg's passphrase-fd argument? - gnupg

Question

I would like to use GnuPG´s decrypt command without any user interation. The script's --passphrase-fd argument seems exactly what I need. But I don't know how it works - haven't found examples.
Could anyone give me an example of such a command, on both Windows and UNIX environments?
(FYI, I'm using GnuPG 2).
Thanks already :)

Answer 1

In order to use the gpg option --passphrase-fd in GnuPG v2, you must specify the --batch parameter. I will first explain how --passphrase-fd works, and then get to the examples.
--passphrase-fd tells GnuPG which file descriptor (-fd) to expect the passphrase to come from. The standard file descriptors are STDIN (0), STDOUT (1) and STDERR (2). For the context of this question, you would normally only be concerned about STDIN (0).
You didn't specify where you want the passphrase to come from, so I will demonstrate the usage of STDIN (standard in) in a variety of ways.
--passphrase-fd 0 tells GnuPG to retrieve the passphrase from input into the current shell; so for example if you want GnuPG to get the passphrase data in the very next line of console input, the command and output would be like so:
gpg2 --batch --passphrase-fd 0 --armor --decrypt /path/to/encrypted_file.pgp
<next line of input is passphrase followed by hitting enter>
gpg: encrypted with 1024-bit RSA key, ID EC18C175, created 2013-10-26
"testkey4321 (4321) <test#4321.com>"
this is a test... this is only a test...
In the above example, the passphrase was provided via file descriptor 0 (STDIN) - which we provided by entering it on the shells current standard input.
In the next example, we will tell GnuPG to retrieve the passphrase from input into the current shell that is actually the output of another command (echo, in this case, which merely "echos" what you tell it to):
echo "mypassphrase" | gpg2 --batch --passphrase-fd 0 --armor --decrypt /path/to/encrypted_file.pgp
gpg: encrypted with 1024-bit RSA key, ID EC18C175, created 2013-10-26
"testkey4321 (4321) <test#4321.com>"
this is a test... this is only a test...
Another example that dumps the contents of a file that contains the passphrase to STDIN -
cat /path/to/file_with_passphrase | gpg2 --batch --passphrase-fd 0 --armor --decrypt /path/to/encrypted_file.pgp
gpg: encrypted with 1024-bit RSA key, ID EC18C175, created 2013-10-26
"testkey4321 (4321) <test#4321.com>"
this is a test... this is only a test...
In summary, --passphrase-fd just tells GnuPG that you want to feed it the requisite passphrase via a standard file descriptor; the difference between GnuPG v2 and GnuPG is merely the --batch parameter.
The above examples should work the same in Windows and *nix environments, with the only difference being that in Windows - depending on your configuration and version - you will have to replace cat with type in order to dump the contents of a file to STDIN.

Answer 2

kylehuff's answer still wouldn't work for me, with gpupg still popping up a password prompt.
According to https://wiki.archlinux.org/index.php/GnuPG#Unattended_passphrase with gnupg version 2.1.0 and higher, you need to do additional steps to support --passphrase-fd
First, edit the gpg-agent configuration to allow loopback pinentry mode:
~/.gnupg/gpg-agent.conf
allow-loopback-pinentry
Restart the gpg-agent process if it is running to let the change take effect.
Second, either the application needs to be updated to include a commandline parameter to use loopback mode like so:
$ gpg --pinentry-mode loopback ...

Answer 3

Using GPG4win/gpg 2.2.3: to use the passphrase-fd 0 and bypass the prompt, I can confirm that the following works:
--pinentry-mode loopback

Answer 4

As I've had to recently figure this out myself I thought it might be worth chiming in.
The answer by kylehuff is very good if you're decryping files, however, if you've need of input/output redirection, such as piping, here's an example of using a non-0 file descriptor to pass the passphrase.
#!/usr/bin/env bash
# Set some variables for easy modding
Var_fd='9'
Var_pass_location="/path/to/passphrase.file"
Var_gpg_decrypt_opts="--passphrase-fd ${Var_fd} --decrypt"
Var_output_location="out.txt"
Arr_string=( "$#" )
# Open file descriptor and shove the passphrase file into it
exec ${Var_fd}<${Var_pass_location}
# Pipe input array though gpg and append to output file
cat <<<"${Arr_string[*]}" | $(which gpg) ${Var_gpg_decrypt_opts} >> ${Var_output_location}
# Do not forget to close the file descriptor
exec ${Var_fd}>&-
Do be warned, outside of special use cases, that saving your private keys passphrase is generally seen as a bad idea or bad security practice. -Also please don't forget to close the descriptor when finished so that your passphrase isn't accessible via that method anymore.- Often I've seen advised in these use cases to use specifically non-passphrase protected keys but that's totally your choose. If you like the above code then you may want to also checkout the script I debugged for key generation either unattended or attended because it covers even less commonly used gpg file descriptor options.
Edits/updates
So I've been debugging the bulk decryption operations and have evidence to show that file descriptors seem to close automatically or perhaps it's auto closed by GnuPG. Check build 152 all the way at the bottom of the raw logs, just before diff checks, you'll find that the first block of encrypted data ate the passphrase leaving the next two blocks of data without a valid passphrase. The related scripts in this operation are ; first the script_decrypt.sh build script sets the test key's passphrase to file descriptor 9 as shown in above examples, then the Helper script is called such that it'll make use of that file descriptor... it's a funky use case but the moral of the story seems to be that what ever bulk decryption operations you plan to implement with GnuPG file descriptors will likely need to follow the steps outlined above as a whole function to properly have the file descriptors reopened. I'll be rewriting the helper script over the next few pushes so check the Travis-CI build logs greater than 152 to find if I've a solution to where file descriptors get closed...
... so that only took two tries to get things working, see the difference in build 154 both the encrypted file and raw input log match. As hypothesised the file descriptors get dumped after first usage by either GnuPG or a sub shell, thus the passphrase needs to be assigned before every decrypt command for bulk decryption to happen.
Hope this was valuable to y'all.